1. Field of the Invention
The present invention relates to a process for producing an ethylene/.alpha.-olefin copolymer of a high-molecular weight with an olefin polymerization catalyst constituted of a metallocene compound, an organoaluminum compound, and an ionizable ionic compound.
2. Description of the Related Art
The low-pressure Ziegler process for polymerization of ethylene or an .alpha.-olefin is well known in the related technical fields. The catalyst for the process is generally prepared by treating a mixture of an organometallic compound or hydride of a metal of Group 1A to 3A of Periodic Table with a compound of a transition metal (Group 3B to 2B of Periodic Table) in a suspension or a solution, or in the absence of a solvent or a diluent.
In recent years, other special kinds of catalysts are being developed which are active in olefin polymerization. Examples of the catalysts are combination of a cyclopentadienyl derivative of a metal such as titanium, zirconium, and hafnium (Group 4B of Periodic Table) with aluminoxane. (See, for example, J. Boor: "Ziegler-Natta Catalyst and Polymerization", Academic Press, New York (1979), and H. Sinn and W. Kaminsky: Adv. Organomet. Chem. 1899 (1980).) These catalysts have ability of forming a stereospecific olefin polymer with high catalyst activity. Japanese Patent Application Laid-Open No. 1-503788 describes a high-pressure high-temperature process for producing ethylene/.alpha.-olefin copolymer by use of a transition metal compound and an aluminoxane as the catalyst.
Nevertheless, such catalysts have not been used industrially mainly because of the following reasons: the aluminoxane cannot readily be produced in a reproducible form, hindering preparation of the catalyst and the polymer with required reproducibility, and the expensive aluminoxane has to be used in a considerably high ratio to the transition metal compound to achieve sufficient activity.
To offset the above disadvantages, Japanese Patent Laid-Open No. 3-207704 discloses ionic metallocene compound prepared by reacting a metallocene with an ionizing ionic compound. PCT Application No. WO 92-1723 discloses a process of .alpha.-olefin polymerization by use of a catalyst system prepared by reacting a halogenated metallocene with an organometallic compound and then bringing the reaction product into contact with an ionizing ionic compound. This catalyst system is advantageous in olefin polymerization. However, when ethylene and .alpha.-olefin are copolymerized by use of such a catalyst system at a high temperature, the resulting copolymer has a low molecular weight disadvantageously.
The inventors of the present invention made comprehensive studies to solve the above problems, and found that an ethylene/.alpha.-olefin copolymer having a high molecular weight is obtained with a high catalyst activity by copolymerizing ethylene with .alpha.-olefin by use of a specific ionic metallocene catalyst at a temperature of not lower than 120.degree. C. The present invention has been accomplished based on the above findings.